Abstract: Numerical simulation and boundary condition sensitivity experiments on the extraordinary rainstorm over Hainan Island duringOctober 4—6，2010 are simulated by the Weather and Research Forecast (WRF) model. The sensitivity experiments show that: using thehighest resolution Global Forecast System (GFS) data as boundary condition can get the best simulation result. The simulation result from 4 km′ 4 km experiment shows: not only the center position of the heavy rainfall but also the rainfall value (1 200 mm) of the simulation is similar tothe observation data (1 084mm); the distribution of 48 h accumulative rainfall amount is successfully simulated which shows the rainfall ineast area is higher than in the west, but the rainfall of simulation in the west is higher than the observation rainfall. Based on the numericalsimulation results, the diagnostic analysis of field such as wind, water vapor flux divergence, pseudo-equivalent potential temperature, lowlevel jet, divergence vertical flux are also performed, the results show that: under effect of tropical depression, cold air and subtropical high,all the existing strong low-lever jet, strong vapor convergence center, atmospheric convective unstable structure, divergence and convergencewith good cooperation of high and low levels had made the upward movement during this event unusually strong, which eventually led to occurringof extraordinary rainstorm.